A regression in Django 1.11.8 made
AuthenticationForm run its
confirm_login_allowed() method even if an incorrect password is entered.
This can leak information about a user, depending on what messages
confirm_login_allowed() raises. If confirm_login_allowed() isn’t
overridden, an attacker enter an arbitrary username and see if that user has
been set to is_active=False. If confirm_login_allowed() is overridden,
more sensitive details could be leaked.

This issue is fixed with the caveat that AuthenticationForm can no longer
raise the “This account is inactive.” error if the authentication backend
rejects inactive users (the default authentication backend, ModelBackend,
has done that since Django 1.10). This issue will be revisited for Django 2.1
as a fix to address the caveat will likely be too invasive for inclusion in
older versions.